One of the major concerns of the footwear industry is constructing shoes which are comfortable. After all, if a shoe is uncomfortable, the purchaser will be unable to wear the shoe and the reputation of the manufacturer will be tarnished. In order to design a comfortable shoe, it is necessary that the shoe designer take into consideration the structure of the foot and the function of particular elements of that structure.
The human foot has to combine the two functions of: (1) supporting the weight of the body while keeping it properly balanced; and (2) propelling the body forward. One element of the foot which is particularly useful in combining the above functions is the longitudinal arch. Unfortunately, the longitudinal arch has largely been ignored by shoe designers.
Referring to FIG. 1, the longitudinal arch A, which is useful for both shock absorption and propulsion, extends from the calcaneum C or heel bone to the heads of the metatarsal bones M of the foot. When standing, the weight of the body is borne by areas of the foot other than the longitudinal arch, causing the longitudinal arch to extend longitudinally, becoming longer and lower to the ground.
During walking, the entire body weight is taken by each foot in turn and is transferred from heel to toe as the center of gravity of the body moves in relation to the ground. The first part of the foot to contact the ground is the heel, and the term used to describe this stage of foot propulsion is "heel strike." The calf muscles then lower the remainder of the foot down in relation to the leg. During the body weight shifting stages of foot propulsion following heel strike, the longitudinal arch acts as a rubber band. That is, it goes through a series of dimensional changes which are both tiring and stressful to the foot.
First, the foot is lengthened, causing the inner longitudinal arch to lengthen and flatten. As the body weight continues to be transferred toward the metatarsal bones, and the muscles of the foot contract, the inner longitudinal arch is raised and shortened. This stage is commonly termed "heel lift-off." In the final stage of propulsion, commonly termed "toe-off," the inner longitudinal arch is then used as a lever to lift the body at the joint of the big toe and propel the foot forward.
Heretofore, various devices have been proposed which are designed to assist the dimensional changes of the inner longitudinal arch during propulsion and to relieve fatigue of the foot. One example of such a device is disclosed in U.S. Pat. No. 634,588 to Roche, which describes an arch support for a shoe. The support comprises a spring plate which is placed upon the insole of the shoe. The plate is curved to conform to the arch of the foot. One end of the plate is attached to the shoe at the heel end. The other end of the plate is freely moveable. When pressure from the foot is applied to and released from the spring, as during walking, the free end of the spring moves along the length of the insole and the spring is allowed to flatten and raise respectively.
While the device of the Roche patent may provide a yielding support for the arch and a portion of the heel of the foot, it suffers from several disadvantages which are overcome by the present invention. For example, the spring of the Roche patent does not begin to flatten until the forefoot has contacted the ground and the body weight has shifted. Therefore, the arch support provided by the Roche spring is not effectuated until the body weight is already positioned over the spring. The arch support of the present invention begins to flatten at heel strike, before the forefoot contacts the ground thereby, providing spring to the foot. Hence, as the longitudinal arch lends spring to the foot so does the arch support of the present invention.
Furthermore, the Roche spring is used in combination with a non-yielding shoe sole which may cause the foot to experience shock when it contacts the ground. In contrast the arch support of the present invention may be used in combination with a yielding shoe sole and therefore, provides added cushioning for the foot. Moreover, in the Roche patent there is no provision of a heel lever which absorbs energy from heel-strike and returns energy to the heel during heel lift-off. Hence, much of the energy generated during heel-strike is lost.
Therefore, the need exists for an arch support which responds to body weight shifting in the same manner as the longitudinal arch of a human foot, i.e., one which begins deflection as soon as body weight transfer has begun. Also, the need exists for an arch support which is provided in combination with a yielding shoe sole so that the shock associated with ground contact is eliminated. Furthermore, the need exists for a heel support which cushions the heel and which absorbs energy from heel-strike and returns that energy to the heel at heel lift-off.